"折合市制" can be translated as "converted to the Chinese municipal system (of units)" or more contextually, when referring to specific measurements like length, weight, etc., you might say "converted to the traditional Chinese municipal measurement units" .
For example:
这个长度折合市制是多少？ (What is this length converted to the traditional Chinese municipal measurement units?)
重量折合市制怎么算？ (How do you calculate the weight converted to the traditional Chinese municipal measurement units?)
In daily usage, if the context is clear, you could also simply say "converted to the municipal system" for brevity, though adding "traditional Chinese" or specifying the type of measurement (e.g., length, weight) can make it more precise.